class_docstr: Selects features from a given stage.
code: def forward(self, x):
      return x[self.stage]
  
file_path: lightly/docs/source/tutorials_source/package/tutorial_pretrain_detectron2.py
imports: import torch
  from detectron2 import config, modeling
  from detectron2.checkpoint import DetectionCheckpointer
  from lightly.data import LightlyDataset
  from lightly.loss import NTXentLoss
  from lightly.models.modules import SimCLRProjectionHead
  from lightly.transforms import SimCLRTransform
label_desc: Use image node_id 2 for calling the SelectStage obj's underlying member method code with example usage: obj.forward(x) and returns: x